Modern Fibre Channel (FC) devices support multiple physical link speeds. Directly connecting FC devices with differing link speeds is not possible. Some multi-speed devices are able to “speed negotiate” when connected together. This allows them to find a mutually compatible link speed. In practice, this means that when a slower speed device is connected to a device (or devices) with a faster link speed, the link operates at the slower speed, even when the fast and slow devices are not communicating with each other. Not all multi-speed devices are able to automatically speed negotiate. Some require user action to manually select link speeds.
Prior art exists in the area of Fibre Channel “Fabrics” and their construction. Fabrics with ports capable of multiple link speeds do allow a slower link rate device or local loop of slower link rate devices to communicate with a faster link rate device or local loop of faster link rate devices by utilising one Fabric port for each device or local loop of devices. However, Fabrics are complex and expensive devices. An example of a Fibre Channel Fabric is disclosed in U.S. Pat. No. 6,205,145.
The following documents concern Fibre Channel Arbitrated Loop topologies (which will be discussed further below) and may provide useful background reading.
U.S. Pat. No. 5,978,379 and U.S. Pat. No. 6,243,386 each disclose the splitting of a loop of devices into two sub-loops in order to improve performance by allowing simultaneous conversations on each half of the loop (bearing in mind that normal loop operation only allows one conversation at any time, resulting in shared bandwidth). The locations of devices on each half of the loop is learnt in real time, i.e. as transfers are initiated.
U.S. Pat. No. 6,324,181 again discloses the concept of splitting a loop into a multiple sub-loops in order to improve performance. In place of the “learning” method used in U.S. Pat. No. 5,978,379 and U.S. Pat. No. 6,243,386, the system disclosed in U.S. Pat. No. 6,324,181 takes control of the loop initialisation process and allocates the AL_PA or loop ids itself. It is noted that this precludes connection to a Fabric, or other device that requires being a “loop master”, as the system in U.S. Pat. No. 6,324,181 must be the loop master itself in order to allocate the loop ids.
U.S. Pat. No. 6,314,488 and WO-A-02/33561 disclose further examples of the concept of splitting a loop into multiple sub-loops.
U.S. Pat. No. 6,118,776 discloses a method and apparatus for allowing loop devices to communicate through a Fabric despite not being able to log in directly with the Fabric.
GB-A-2363040 discloses a method for link speed negotiation in a Fibre Channel Arbitrated Loop.